Chemical separations are used in industry, research and medicine for purposes of analysis, research, quality control and diagnosis. One type of chemical separation is chromatography. Chromatographic separations use differences in the affinity of compounds to different media to pull one compound from others. For example, food coloring is often sold as a mixture of compounds in water which impart a color to foods and the like to which they are added. If one places a drop of the food coloring on a paper towel, the food dye spreads through the towel. The paper towel acts as a fixed medium through which the food coloring flows. Upon drying, the drop of food coloring will exhibit bands of different colors representing the different compounds. These compounds have separated in a process based on differences in affinity to the fixed media.
Chromatographic separations are routinely performed with gas samples or samples that are made into gases upon analysis or liquid samples which remain liquid. Separations performed on gas samples are known as gas chromatography, sometimes abbreviated as “GC”. Chromatographic separations performed with liquid samples are known as liquid chromatography, or simply, “LC”.
Liquid chromatography performed under pressure as fluid flows through a fixed media is known as high performance or high pressure liquid chromatography, or simply, “HPLC”. HPLC is routinely performed with a substantially closed system in which a sample is injected into the flow of a fluid moving in a conduit. The fluid containing a sample is directed into a column or cartridge containing a fixed media. For the purpose of this application, the terms column and cartridge are interchangeable. A column is a cylindrical form through which fluid can flow. The column contains a fixed medium, usually a packing of particles, or a monolithic porous material around which the fluid flows.
Each compound in the sample exhibits a characteristic affinity to the fixed medium. Compounds with lower affinity to the medium will exit the column ahead of compounds with higher affinity to the media. The presence of the compounds is often detected by a detector such as a mass spectrometer, optical systems, electrochemical systems or other physical-chemical detection means known in the art.
Analytical chemists and medical researchers often are looking for a particular compound, series of compounds or a compound that has not been previous characterized. Such compound may suggest a disease state, or such compound may suggest the presence of a drug or drug metabolyte. Huge numbers of samples are evaluated using chromatographic processes. These processes become standardized and routine.
A typical process may comprise a loading step, an elution step and one or more, often three to four, wash and/or reconditioning steps. As used herein, a loading step refers to a step in which a sample is introduced into a column or cartridge. An elution step refers to a step in which one or more compounds having affinity to the media in a column are compelled to leave the column. Elution is performed by introducing a change in the fluids flowing through the column. Typically, these changes can comprise a change in hydrogen ion concentration (pH), ionic strength, or solvent/solution changes. The elution step is an important step in analytical processes. The elution step is usually associated with a detecting the presence or absence of a compound. Detectors can be expensive and it is desirable to use detectors efficiently. Washing refers to the process of removing materials from the immobilized phase that are not desired. Reconditioning refers to returning the media to a condition for receiving a further sample. Washing and reconditioning steps are not normally associated with meaningful signals from a detector.
The term “step period” refers to the time in which a step is performed. The longest step period of a multi-step process is the largest period of time for any of the steps of the process. The total process time is the sum of all the work periods associated with a process. And, where different steps are being performed at the same time, the total process time will be the number of steps times the longest step period. The term, “in parallel” refers to in the same work period.
It would be desirable to perform different steps of a multi-step process during a single step period, through multiple step periods, to produce a constant flow of sample eluted from the chromatography media to a detector. That is, the detector will receive the fluid from an elution step for substantially each step period.